1. Field of the Invention
The present invention relates to a magnetic recording/reproducing apparatus such as a VTR (Video Tape Recorder), and in particular, to a technique for correcting a distortion called a skew on a picture generated by switching the heads, when scanning a magnetic tape helically with a plurality of heads.
2. Description of the Related Art
A VTR of rotary a 2-head helical scan system reproduces a signal stored on a magnetic tape utilizing the two heads provided on the circumference of a rotary drum. The two heads alternately scan the magnetic tape and pick up the recorded video signals. There exist discontinuation in the video signals reproduced by each of the heads. However, the discontinuous parts are removed from the video signals by connecting the video signals reproduced with the two heads by means of an operation called head switching.
FIG. 1 is a timing diagram of the head switching of a VTR of rotary 2-head helical scan system. Referring to FIG. 1 (a), when the head switching pulse is at its high level, a video signal picked up with the first head is employed. When the head switching pulse is at its low level, a video signal picked up with the second head is employed. The point at which the head switching pulse changes its level from the high level to the low level, (i.e. the point in which the video signal is switched from the one picked up with the first head to the one picked up with the second head), is called a head switching point.
If the heads are ideally switched, the obtained video signal should not include any noise caused by switching the heads. In practice, however, a gap or an overlap due to the head switching takes place in the video signal.
FIG. 1 (b) is a waveform diagram of a reproduced video signal. FIGS. 1 (a) and 1 (b) is depicted in accordance with the same time base. Referring to FIG. 1, a gap or an overlap takes place in the video signal at the head switching point. The gap or the overlap of the video signal appears on the reproduced picture as a video distortion called a skew.
The position of the head switching point depends on the rotating speed of the rotary drum and the tape running speed. The rotation of the rotary drum and the running of the tape are controlled so as to be substantially constant by an exclusively prepared servo system. The control of the servo system, however, includes jitters resulting in the constant fluctuation of the switching point.
A method for correcting a skew utilizing a field memory is disclosed in "ITEJ (Institute of Television Engineers of Japan) GIHO (Technical Report)" (Vol. 13, No. 50 October, 1989), pp. 7-9. The method is called "line shuffling".
According to the line shuffling method, a signal for one field is once stored in a field memory. The scan lines of the video signals stored in the field memory are extracted every predetermined number thereby forming one segment signal. The rest of the scan lines are also divided into other segment signals in the same manner. Each of the divided segment signals includes rough information over the entire picture, although the number of the scan lines included therein is insignificant. Each segment signal is stored on one track on a magnetic tape.
The above-mentioned article describes that a great improvement was provided to the reproduced picture, in case a signal for one field is divided and stored on a plurality of tracks by utilizing the shuffling method.
However, if thus trying to correct skews by utilizing a field memory, complicated circuitry is required. Furthermore, it is well known that the field memory is expensive, and such a use of the field memory will push up the cost of a product.
On the other hand, a method for correcting the time base shift of a reproduced video signal is disclosed in ITCJ GIHO (Vol. 35, No. 6 1981) pp. 37-44. The method detects a reference signal such as a horizontal synchronizing signal or a burst signal included in a video signal and shifts the time base on the detected signal.
The method, however, merely performs the time base shift of a video signal or the correction of jitters etc., and therefore, is irrelevant to skews appearing on a picture. The method cannot deal with the skews on the picture which are caused by the head switching at the time of reproducing.